CLEARWATER — Rallying supporters, transit advocates launched a campaign on Friday to pass a 1-cent sales tax increase in Pinellas that would pay for a massive overhaul of the county's transportation system.

"This is vitally important," Pinellas County commissioner Ken Welch said, speaking before a room packed with elected officials, business leaders, political consultants, and activists, many of whom wore green pins that declared only "Yes."

"It's about jobs, it's about economic development, it's about making us competitive not only with Polk County … but with Orlando, Miami and regions around the United States and internationally," he said.

Held in an office building off Ulmerton Road, where ongoing construction and traffic jams serve as a daily reminder of Pinellas' transportation problems, the campaign kickoff marked the formal beginning to an effort that will stretch from now until Nov. 4, when the Greenlight Pinellas referendum goes before voters.

The campaign, called "Yes for Greenlight," is being led by Ronnie Duncan, chairman of the Tampa Bay Area Regional Transportation Authority, and Chris Steinocher, president of the St. Petersburg Chamber of Commerce. In place of a traditional campaign manager, it will be run by Tucker Hall, a Tampa-based public relations firm, which has put vice president Keith Rupp in charge of daily operations.

Over the next nine months, the campaign's aim is to persuade Pinellas residents to support the transit referendum, which calls for replacing the property tax that currently funds the county's transit agency with a 1-cent sales tax, a substitution that would increase the agency's annual budget by roughly $100 million.

With that additional money, the agency would oversee a complete restructuring of the bus system, increasing service by 65 percent and doubling the fleet. The plans calls for six bus rapid transit routes along some of the county's main corridors, as well as more routes to Tampa. It also includes plans to build 24 miles of light rail connecting Clearwater and St. Petersburg.

Joining a ballot crowded with a gubernatorial race and an initiative to legalize medical marijuana in Florida, the transit referendum will have to compete for attention. But its supporters also believe the measure will benefit from a political tailwind — the combination of a likely increase in voter turnout and the potential for marijuana to motivate more young people to vote.

Advocates say they are cautiously optimistic. According to Duncan, the group commissioned a private poll last month that found that roughly 64 percent of Pinellas voters support the Greenlight plan, a figure that significantly exceeds what earlier polls have shown. In December, a poll paid for by the Tampa Bay Times and other local news outlets found that 55 percent of Pinellas voters would back the referendum, 36 percent would oppose it, and 9 percent were unsure.

Not yet fully formed, the "Yes for Greenlight" campaign has only begun to court donors, Duncan said, and has not amassed a war chest to pay for months of advertising. It has set up a tax exempt advocacy organization, a 501c4, he said, a step that will allow it to collect contributions and keep funders anonymous.

Though it was three years ago that Hillsborough voters soundly defeated a similar referendum, Duncan said there's been "no hesitation" among business leaders to get involved in Pinellas.

"When Hillsborough did this, their plan wasn't even ready for prime time until two months before the election," he said.

The fact that advocates in Pinellas can point to their plan's bus and rail maps — and that most of the county's political establishment is on board — has put contributors at ease, he said.

The referendum's main opposition group, No Tax for Tracks, has raised roughly $20,000, its leader Barbara Haselden said on Friday.

"You don't need a lot of money to defeat this," she said, predicting that for every dollar her group takes in, referendum supporters will have to raise $1,000.

Organizers would not say how much they expect to raise, but similar campaigns in other cities have cost in the range of $1 million to $2 million.

"Yes for Greenlight" will likely rely on direct mail, as well as social media. The campaign plans to focus its efforts heavily on undecided voters and on increasing turnout among supporters.

"You're not going to see us spending money on a glitzy TV campaign, we don't think we need that," Steinocher said.